#include <iostream>
using namespace std;
typedef unsigned short int USHORT;
USHORT xvalue, yvalue;
USHORT doublex (int, int); // function prototype
int main (){
	cout << "Please enter your value for x\n";
	cin >> xvalue;
	doublex (xvalue, yvalue);
	cout << yvalue;
	return yvalue;
}

USHORT doublex (USHORT xvalue)
{
	USHORT yvalue;
	yvalue = (xvalue * 2);
	return yvalue;

}

COMPILES FINE
WHEN LINKING I GET...
--------------------Configuration: functions - Win32 Debug--------------------
Linking...
hello.obj : error LNK2005: _main already defined in main.obj
hello.obj : error LNK2005: "unsigned short __cdecl doublex(unsigned short)" (?doublex@@YAGG@Z) already defined in main.obj
hello.obj : error LNK2005: "unsigned short yvalue" (?yvalue@@3GA) already defined in main.obj
hello.obj : error LNK2005: "unsigned short xvalue" (?xvalue@@3GA) already defined in main.obj
main.obj : error LNK2001: unresolved external symbol "unsigned short __cdecl doublex(int,int)" (?doublex@@YAGHH@Z)
hello.obj : error LNK2001: unresolved external symbol "unsigned short __cdecl doublex(int,int)" (?doublex@@YAGHH@Z)
LIBCD.lib(wincrt0.obj) : error LNK2001: unresolved external symbol _WinMain@16
Debug/functions.exe : fatal error LNK1120: 2 unresolved externals